The invention to a gearbox of an agricultural or industrial vehicle, in particular, an agricultural tractor.
It is known practice to equip utility vehicles with parking locks. The parking lock is to be applied during parking or stationary use in order to prevent the vehicle from rolling away. This can be attained, for example, by a locking lever mechanism, which applies a positive lock to a set of gear teeth connected to a shaft so as to prevent rotation of the shaft in the parking position.
DE-PS-36 36 978 shows a parking lock for an automatic transmission, in particular, in connection with utility vehicles. Here a parking gear, in a one-piece configuration with an output shaft, is locked by means of a locking arm so that rotation of the output shaft during parking is not possible.
The lever that operates the parking lock may be configured as an independent operating element. However, agricultural tractors are known in which the operation of the parking lock is integrated into a gearshift lever which is used to select various gear ratios. Furthermore a second lever permits the selection of the direction of operation. This so-called reverse gearshift lever is preferably attached to the steering column within reach of the left hand. It permits shifting between a forward, neutral or reverse position.
With the use of a parking operating lever and a separate gear ratio or vehicle direction selector lever, the gear ratio or the vehicle direction selector lever is not inevitably forced into its neutral position, when the parking lock is engaged and the vehicle is stopped. Therefore it is possible that during a renewed starting of the vehicle the operator overlooks the fact that a gear ratio is already engaged or a selection of the direction of operation has been made. If the operator now disengages the parking lock, the vehicle begins to move forward or in reverse, unintended and unexpected by the operator. This can lead to dangerous situations.
U.S. Pat. No. 4,246,989 shows an arrangement for the locking of a gearbox operating element and a conventional brake with brake linings that can be actuated by a hydraulic cylinder and that open against the force of a spring when pressure is applied. The gearbox operating element can be moved axially for shifting between two opposite directions of operation and a neutral position and extends within a housing provided with a stop with axially opposed end faces. Alongside the stop a scissors-shaped arrangement is located that consists of two angular levers, the points of the knees of which are supported in joints in the housing. The ends of each of the first leg of the angular levers are connected to each other in a joint by a connecting pin. The second legs of the angular levers extend approximately parallel to each other and are drawn towards each other by a extension spring, so that they are in contact on both sides with the end faces of the stop and force the gearbox operating element into its central neutral position. To release the gearbox operating element a piston arrangement is provided which is actuated by the brake pressure. With increasing brake pressure the piston is moved and presses against the connecting joint of the two first legs. Thereby the angular levers are pivoted so that the scissors open against the force of the spring whereupon the two second legs move away from one another. This permits a free movement of the gearbox operating element.
The known locking mechanism is appropriate only for an application with a hydraulically actuated brake. It exhibits a relatively expensive configuration. Furthermore it is possible to move the gearbox operating element against the force of the extension spring out of its neutral position, which could be a disadvantage for applications with a parking lock.